1. Field of the Invention
This invention relates to a charge control agent useful for controlling triboelectricity of a toner for developing an electrostatic latent image. The present invention is also directed to a toner for use in developing an electrostatic latent image in an image forming method such as electrophotography, electrostatic recording and electrostatic printing and to a developing device using the developer.
2. Description of the Prior Art
Methods for developing an electrostatic latent image to form a toner image include a method using a one-component type developer composed only of a toner and a method using a two-component type developer composed of a toner and a carrier which are charged in opposite polarity. The one-component type method may be, for example, a powder cloud developing method in which toner particles are sprayed over an electrostatic latent image-bearing surface, a contact developing method (touch down method) in which a latent image-bearing surface is directly contacted with a bed of toner, or a dielectric developing method in which a latent image-bearing surface is contacted with a magnetic, electrically conductive toner. The two-component type method may be, for example, a magnet brush method using iron powder as a carrier, a cascade method in which glass beads are used as a carrier or a fur brush method in which a fiber brush is used as a carrier.
In the above developing systems, fine particles each composed of a matrix of a synthetic or natural binder resin, such as a polystyrene resin, and a colorant, such as carbon black, dispersed in the matrix are generally used as a toner. Such a toner is finely divided into a particle size of, for example, 1-30 μm. In the case of a magnetic toner, a magnetic material such as magnetite is incorporated into the above toner.
Toner to be used for developing an electrostatic latent image is positively or negatively charged according to the polarity of the electrostatic latent image to be developed. Because the amount of charges generated by using the triboelectricity of the binder resin is small, a charge control agent is generally incorporated into the toner to obtain clear images.
Known charge controlling agents include metal complexes of monoazo dyes; humic or nitrohumic acid or a salt thereof; metal complexes (e.g. Ni, Co, Cr, and Fe metal complexes) of aromatic hydroxycarboxylic or dicarboxylic acids such as salicylic acid, naphthoic acid and dicarboxylic acid; sulfonated copper phthalocyanine pigments; nitrified or halogenated styrene oligomers; chlorinated paraffins; and melamine resins. The known charge controlling agents, however, suffer from one or more defects such as non-uniformity in triboelectricity and susceptibility to decomposition during kneading. Further, the use of a charge control agent containing heavy metals such as chromium, nickel, chromium and zinc has environmental problems and safety problems against human bodies.
JP-A-H01-306861 discloses the use of an organic boron compound as a charge control agent. JP-A-S61-3149 discloses the use of a bisphenol compound as a charge control agent. These charge control agents are, however, hygroscopic in nature and are apt to cause a change in their triboelectricity. Additionally, they do not have highly satisfactory triboelectricity.
Incidentally, a polyester resin or an epoxy resin is now increasingly used as a binder resin for toners because of their advantages that these resins do not adversely affect the inherent color of colorants thereof, that these resins have good resistance to tackiness to polyvinyl chloride mats and that these resins have good preservation stability and yet permit low temperature fixation. When these binder resins are used in conjunction with the above known charge control agents, sufficiently high triboelectricity cannot be obtained or the triboelectricity gradually reduces even when the initial triboelectricity is high, so that problems of fatigue or toner dispersion are apt to be caused. Probably, the problems are attributed to the presence of —COOH and —OH functional groups in these resins which groups would prevent maintenance of stable triboelectricity of the charge control agents.
Conventionally, an oil is applied to a heated roll to prevent “hot offset” which is a phenomenon occurring in fixation of toner image on paper with a heated roll and refers to deposition of fused toner onto the heated roll. The use of the oil requires the installation of an oil tank and hinders compactness of the apparatus. Recent trend is toward incorporation of a wax into the toner for improving releasability. The use of wax, however, reduces the fluidity of the toner particles and, hence, it becomes difficult to obtain desired amount of charge in a moment by friction. Since the frictional contact time in the case of a one-component type toner is much shorter than that for a two-component type toner, there is a great demand for a charge control agent having excellent triboelectricity.
Additionally, there is an increasing demand for white or colorless charge control agents which can be suitable used for color toners. Currently available white or colorless charge control agents, however, do not have good charging characteristics or are very expensive.